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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorgyrogear
    • CommentTimeAug 14th 2018
     
    Ah, OK, I stand corrected ! Capillary Action Damp, then :shamed:

    gg
  1.  
    "Hmm, the overflow I am talking about is from the (nominally cold) header tank of my thermal store. Sorry, I misunderstood your situation. Perhaps/probably the rules are different for your system."
    Hi @djh, my overflow is also the overflow from the cold header / expansion tank for my open vented thermal store.
    •  
      CommentAuthordjh
    • CommentTimeAug 15th 2018
     
    Posted By: ComeOnPilgrim"Hmm, the overflow I am talking about is from the (nominally cold) header tank of my thermal store. Sorry, I misunderstood your situation. Perhaps/probably the rules are different for your system."
    Hi @djh, my overflow is also the overflow from the cold header / expansion tank for my open vented thermal store.

    Looks like I misunderstood again, then. Various people in the thread are discussing other types of F&E tank. So your situation is the same as mine. My tank is plastic, supplied by the thermal store manufacturer for the purpose, and the overflow goes via plastic pipe to a plastic tundish into plastic drains, all inside the house. The tank is insulated with polythene-wrapped fibreglass/rockwool, also supplied by the manufacturer.

    Posted By: djhThat was my original plan, but my plumber said such an arrangement was more of a nuisance than a ballcock and I went with his suggestion. It's too early to say whether he was right - we haven't had any problems yet.

    But FWIW, since I wrote this we have had a problem. The ballcock failed (fell off the end of its arm) and in traditional murphy fashion, it did it whilst I was away for the day. My wife noticed the noise but couldn't identify it's exact source so she turned the water mains off. When I returned I managed to identify the source and isolated the feed to the tank. I replaced the ballcock a few days later.
    • CommentAuthorCWatters
    • CommentTimeAug 15th 2018
     
    This place says they make custom hot water tanks and expansion tanks in copper or SS...

    http://www.newarkcoppercylinder.co.uk/header-tanks.html
  2.  
    Thanks @djh! Mine is copper. It doesn't get hot, even if the store is really hot (90°), but if it boiled, presumably is there a risk that the hot would spurt into the expansion tank?
  3.  
    Just to return to this - is an overflow with a tundish needed if there is only a manual fill, not a ballcock? I could see why you would need a tundish to tell you when the ballcock is broken and filling it up with water, but presumably a manual fill would never overflow (except when you were manually filling it), even if the store was boiling?
    •  
      CommentAuthordjh
    • CommentTimeAug 16th 2018
     
    I think you're absolutely right, as long as you don't overfill the expansion tank. What the rules say, I don't remember.

    I suppose the only case where there could be an overflow would be if there were a boiler coil or a DHW coil in the thermal store, since holes in those could cause them to inject water into the store.
  4.  
    Hi @djh, what rules would be relevant here? Perhaps I could look them up?
    •  
      CommentAuthordjh
    • CommentTimeAug 21st 2018 edited
     
    The Water Regulations. https://www.wras.co.uk/consumers/advice_for_consumers/what_are_the_water_regulations_/

    edit: I think the guides assume that there is always an inlet to a tank, so you might need to call WRAS to ask what they think is necessary when there isn't.

    edit2: I think any overflow must include or terminate with an air gap. So the question is whether there needs to be an overflow if there is no inlet?
  5.  
    There was a very tragic accident some years back where a hot cylinder overheated due to a faulty thermostat and boiled over, the expanding water and steam escaping via the overflow for the cylinder running into the cold header tank.

    The header tank filled up with hot water and being plastic it softened and eventually collapsed, dumping scalding water onto the little girl in the room below.

    The overflow from both the cylinder/store and its header tank should be arranged to avoid hot water accumulating in the header tank.

    http://www.dailymail.co.uk/news/article-506604/Water-tank-fault-scalded-baby-girl-death-known-unreliable.html

    Edit: maybe obvious but when the cylinder or store boils, the volume of steam bubbles that are generated at the heating surface, displace an equal volume of boiling water, which has to overflow to somewhere safe.
  6.  
    As a comment on the tragedy, the article stated that 'hot water was coming out of the cold tap' to me this implies that the cold water storage tank doubled up as a F&E tank for the DHW tank. I don't have a problem with that - except that in my book a F&E tank for a DHW should not be plastic - because in he event of a fault they can soften an deform, which it appears happened in this case. Perhaps had the cold tank been metal the tragedy would not have happened!
  7.  
    Hi Peter, that's a common setup where the cold supply to bathrooms comes from the hot cylinder header tank as then hot and cold are equal pressure and shower mixers work better. Header tanks and plumbing are often plastic, implying that we rely on the thermostats, and on the householder to notice if there's a problem.

    Hi CoP, just to emphasise, the overflow is a safety feature that is needed irrespective of your filling arrangement. It needs an air break (tundish) to avoid syphoning and to alert you to problems such as boiling.
  8.  
    Posted By: WillInAberdeenHi CoP, just to emphasise, the overflow is a safety feature that is needed irrespective of your filling arrangement. It needs an air break (tundish) to avoid syphoning and to alert you to problems such as boiling.

    The tundish won't provide an air break to avoid syphioning because the tundish will be below the level of the tank water level and so syphioning would continue. In any case you don't need to break any syphioning in an overflow as it will automatically break when the water level drops below the overflow level (and before that time flow is needed). IMO the only reason for a tundish is to give a visual indication of flow in the overflow.
    •  
      CommentAuthordjh
    • CommentTimeAug 22nd 2018 edited
     
    Posted By: Peter_in_HungaryIMO the only reason for a tundish is to give a visual indication of flow in the overflow.

    That's one reason, indeed. The other reason for an air gap is to prevent any possibility of backflow FROM the drain, not to it. There is no absolute requirement for a tundish; an overflow pipe that terminates in the open air is an alternative. The tundish is only required as an airgap where the overflow joins a drain.

    WillInAberdeen, do you have a link to the regulation that requires the overflow, please?
  9.  
    Link is as you posted, no?
    Posted By: djhThe Water Regulations.https://www.wras.co.uk/consumers/advice_for_consumers/what_are_the_water_regulations_/" rel="nofollow" >https://www.wras.co.uk/consumers/advice_for_consumers/what_are_the_water_regulations_/
      Screenshot_20180822-213646.png
    •  
      CommentAuthordjh
    • CommentTimeAug 23rd 2018
     
    Hmm, what is that document?

    The law seems to require overflows for a “storage cistern”, which are a distinct class that are not a “combined feed and expansion cistern”.

    http://www.legislation.gov.uk/uksi/1999/1148/contents/made
  10.  
    Posted By: ComeOnPilgrimJust to return to this - is an overflow with a tundish needed if there is only a manual fill, not a ballcock? I could see why you would need a tundish to tell you when the ballcock is broken and filling it up with water, but presumably a manual fill would never overflow (except when you were manually filling it), even if the store was boiling?

    Posted By: djhThe law seems to require overflows for a “storage cistern”, which are a distinct class that are not a “combined feed and expansion cistern”.

    So IMO an over flow with a tundish would be of no advantage with a F&E tank with manual fill only, but an over flow would be needed - going to the nearest convenient frost free drain. (i.e. not poked out of the eaves to the gutter).
    syphoning would not be an issue as any syphon would be broken as soon as the water level fell below the overflow level and with no auto-fill this would be quite soon. Back flow would not happen, or at least if there was enough pressure in the drain to push drain contents up into a F&E tank then back flow to this tank will be the least of the problems.

    Why the concern? Do what is reasonable which IMO is a straight forward overflow connected to a drain. Fill to the point of over flow and any expansion goes down the over flow and the system level self regulates. (until the next top-up) If you have inhibitor in the system then fill to a point that normal use will not over flow.

    If you need to know because of impending inspection requirement then ask the person who will do the inspection and follow what (s)he says. Don't ask the inspectors colleague as you may well get differing stories.
  11.  
    Hi DJH, screenshot is from your WRAS link, follow DEFRA Guidance.
    The definitions section of the regs maybe helps clarify?
    It subdefines "cisterns" into: "flushing cistern" and "storage cistern [which is any] cistern that is not a flushing cistern."

    It further subdefines "expansion cistern" and "combined feed and expansion cistern", which are types of storage cistern.

    The subsequent regs covering all the various kinds of "cisterns" each require an overflow / warning, except for those little ones that flush urinals, which TBH all seems pretty reasonable/obvious.

    +1 to everything Peter said, except I'd want to see if it was overflowing. Some people don't like pipes sticking through their thermal envelope.
    •  
      CommentAuthordjh
    • CommentTimeAug 23rd 2018
     
    Posted By: Peter_in_HungarySo IMO an over flow with a tundish would be of no advantage with a F&E tank with manual fill only, but an over flow would be needed - going to the nearest convenient frost free drain. (i.e. not poked out of the eaves to the gutter).

    Direct connection definitely not allowed. The overflow must either terminate in free air (outside, or perhaps above a WC pan) or it must have an airgap (i.e. tundish) in the connection to a drain.
    •  
      CommentAuthordjh
    • CommentTimeAug 23rd 2018
     
    Posted By: WillInAberdeenThe definitions section of the regs maybe helps clarify?
    It subdefines "cisterns" into: "flushing cistern" and "storage cistern [which is any] cistern that is not a flushing cistern."

    It further subdefines "expansion cistern" and "combined feed and expansion cistern", which are types of storage cistern.

    Not in my copy it doesn't. Page 8 has the definitions, of which the relevant bits are:

    “cistern” means a fixed container for holding water at atmospheric pressure;

    “combined feed and expansion cistern” means a cistern for supplying cold water to a
    hot water system without a separate expansion cistern;

    “flushing cistern” means a cistern provided with valve or device for controlling the
    discharge of the stored water into a water closet pan or urinal;

    “storage cistern” means a cistern for storing water for subsequent use, not being a
    flushing cistern;

    And in fact later goes on to separate the two:

    16 - (2) Every inlet to a storage cistern, combined feed and expansion cistern, WC flushing cistern
    or urinal flushing cistern shall be fitted with a servicing value on the inlet pipe adjacent to the
    cistern.

    and then requires an overflow only in storage cisterns:

    16 - (4) Every storage cistern shall be fitted with–
    (a) an overflow pipe, with a suitable means of warning of an impending overflow, which
    excludes insects;

    So again, I suggest that talking to WRAS is probably the best way to answer the question. Personally, I would just fit an overflow with an airgap and have done with it, but it's COP's question.
  12.  
    That's what I just quoted!
    “storage cistern” means a cistern for storing water for subsequent use, not being a flushing cistern


    My header tank does store water, but it doesn't flush anything, so it's a storage cistern. If it did flush something it would be a flushing cistern. Either way it is required to have a visible overflow, same as all the millions of other ones do, because nobody wants water overflowing through the ceiling, particularly in the event that the system is boiling.
    •  
      CommentAuthordjh
    • CommentTimeAug 23rd 2018
     
    Posted By: djhagain, I suggest that talking to WRAS is probably the best way to answer the question
  13.  
    Thanks all! I've set out my questions in detail for WRAS and will post their response here.
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